The International Journal of Biochemistry & Cell Biology 63 (2015) 16–20 Contents lists available at ScienceDirect The International Journal of Biochemistry & Cell Biology jo urnal homepage: www.elsevier.com/locate/biocel Organelles in focus Mitochondria: Much ado about nothing? How dangerous is reactive ଝ oxygen species production? a,b a,b,∗ Eliskaˇ Holzerová , Holger Prokisch a Institute of Human Genetics, Technische Universität München, Munich, Germany b Institute of Human Genetics, Helmholtz Zentrum München, Neuherberg, Germany a r a t b i c s t l e i n f o r a c t Article history: For more than 50 years, reactive oxygen species have been considered as harmful agents, which can attack Received 31 October 2014 proteins, lipids or nucleic acids. In order to deal with reactive oxygen species, there is a sophisticated Received in revised form 20 January 2015 system developed in mitochondria to prevent possible damage. Indeed, increased reactive oxygen species Accepted 29 January 2015 levels contribute to pathomechanisms in several human diseases, either by its impaired defense system Available online 7 February 2015 or increased production of reactive oxygen species. However, in the last two decades, the importance of reactive oxygen species in many cellular signaling pathways has been unraveled. Homeostatic levels were Keywords: shown to be necessary for correct differentiation during embryonic expansion of stem cells. Although Reactive oxygen species the mechanism is still not fully understood, we cannot only regard reactive oxygen species as a toxic ROS scavenging by-product of mitochondrial respiration anymore. ROS signalization This article is part of a Directed Issue entitled: Energy Metabolism Disorders and Therapies. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Key facts Organelle facts • • Reactive oxygen species are produced in various cell com- Mitochondria produce vital energy in the form of ATP via partments. oxidative phosphorylation. • • Previously thought of as harmful agents only, they are now Mitochondria have their own genome, called mitochondrial considered as important signaling molecules with potential DNA. • therapeutic effect. Mitochondria are responsible for most of the reactive oxygen species via oxidative phosphorylation. • Mutations in nuclear encoded genes of mitochondrial proteins potentially result in inherited diseases, with an inci- dence of 1 in 10,000, most of them causing neuropathies or myopathies. • Mitochondrial diseases of oxidative phosphorylation can be connected with increased ROS • Mitochondria have their own ROS defense system. Abbreviations: I, complex I; II, complex II; III, complex III; IV, complex IV; ␣- KGDH, ␣-ketoglutarate dehydrogenase; AO, alternative oxidase; cyt, cytochrome; DHODH, dihydroorotate dehydrogenase; ETF, electron transfer flavoprotein; GLRX, 1. Introduction glutaredoxin; GPx, glutathione peroxidase; GSH GSSG, glutathione; GSR, glu- tathione reductase; mGPDH, mitochondrial glycerophosphate dehydrogenase; The discussion about reactive oxygen species (ROS) started MAO, monoamine oxidase; NADH DH, external NADH dehydrogenase; NOS, nitric around the year 1956 (Harman, 1956) with the finding that 2% of oxide synthase; OXPHOS, oxidative phophorylation; PDH, pyruvate dehydroge- nase; PRXIII, peroxiredoxin III; ROS, reactive oxygen species; TXN2, thioredoxin 2; the oxygen which is used up by the respiratory chain in mitochon- TXNRD2, thioredoxin reductase. dria can be released and transformed into a superoxide radical ଝ This article is part of a Directed Issue entitled: Energy Metabolism Disorders and •− anion O2 by consuming a single electron coming from the respira- Therapies. ∗ tory chain. Traditionally, most of the ROS production is believed to Corresponding author at: Institute of Human Genetics, Technische Universität originate from the electron transport chain in mitochondria, espe- München, Munich, Germany. Tel.: +49 8931872890. E-mail address: [email protected] (H. Prokisch). cially from complexes I and III. Later on, many other proteins were http://dx.doi.org/10.1016/j.biocel.2015.01.021 1357-2725/© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). E. Holzerová, H. Prokisch / The International Journal of Biochemistry & Cell Biology 63 (2015) 16–20 17 Fig. 1. Sites of ROS production. Many different sites of ROS production exist within a cell. Most of them are located in the mitochondrial environment such as the complexes of the respiratory chain: complex I (I), complex II (II), complex III (III), or mitochondrial glycerophosphate dehydrogenase (mGPDH) next to ␣-ketoglutharate dehydrogenases (␣-KGDH), electron transfer flavoprotein (ETF) and ETF ubiquinone oxidoreductase, pyruvate dehydrogenase (PDH), aconitase, alternative oxidase (AO), complex IV (IV), Shc dihydroorotate dehydrogenase (DHODH), external NADH dehydrogenase (NADH DH), protein p66 , cytochrome (cyt) b5 reductase, monoamine oxidase (MAO) and nitric oxide synthase (NOS). Other proteins or organelles can also contribute to ROS production. Respiratory chain complexes are displayed in blue, other ROS contributors in green, organelles in violet. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.) described as potential ROS producers, but the exact contribution formation, is the mitochondrial glycerolphosphate dehydroge- from different sites is not yet fully understood. Many ROS pro- nase (Drahota et al., 2002). It is located in the inner membrane ducers arise with disruption of cell homeostasis, but, in contrast, facing the intermembrane space. Another significant contribu- several proteins produce ROS to restore this homeostasis. Here, we tion to ROS production occurs during fatty acid oxidation due to summarize sites of reactive oxygen species production and mito- electron transfer flavoprotein (ETF) that accepts electrons from chondrial defense mechanisms and focus on described roles of ROS different dehydrogenases and transfers them through its mem- in cell signalization as a beneficial, yet often overlooked effect of brane partner ETF ubiquinone oxidoreductase to the coenzyme Q ROS in cell metabolism. pool in the inner membrane (Ruzicka and Beinert, 1977). Next, there is a multisubunit pyruvate dehydrogenase complex (Starkov et al., 2004) and a structurally similar membrane bound enzyme 2. Organelle function: mitochondrial sites of ROS production complex of ␣-ketoglutarate dehydrogenase (␣-KGDH) which has been proposed as a source of superoxide and hydrogen perox- + ide under low availability of NAD , the natural electron acceptor Mitochondria play a key role in aerobic cellular metabolism. The of ␣-KGDH (Starkov et al., 2004). Many of the aforementioned incomplete oxidation of oxygen to water results in superoxide pro- proteins contain flavin in their active site, which is directly inter- duction, virtually ROS. Even though it is still unclear if ROS are only acting with electrons and plays a possible role in the electron harmful or beneficial, many ROS producing sites were described leakage. (Fig. 1). However, an exact contribution of each enzyme is not yet Aconitase, an enzyme in the mitochondrial matrix, is able to known. Complexes of the respiratory chain in mitochondria are transform hydrogen peroxide into hydroxyl radicals during a Fen- considered as main producers, especially complex I in several sites ton reaction with its iron–sulphur cluster. Aconitase, though, is of the enzyme (Koopman et al., 2010), complex III in subunits inter- easily inhibited by the presence of superoxide (Vasquez-Vivar et al., acting with coenzyme Q (Raha et al., 2000; Turrens et al., 1985) and 2000). The function of many proteins is changed upon oxidative complex II under low substrate conditions (Quinlan et al., 2012) as well. stress in cells. Redox disbalance and subsequent oxidation of, for Shc example, protein p66 , which plays an important role in the reg- Within mitochondria, minor ROS producers can also be found. ulation of apoptosis, translocates it into the intermembrane space First of all, one protein, which is able to transfer electrons to produce H O (Pelicci, 2005). to the coenzyme Q pool as well as to contribute to the ROS 2 2 18 E. Holzerová, H. Prokisch / The International Journal of Biochemistry & Cell Biology 63 (2015) 16–20 There are several other proteins contributing to mitochon- Due to indirect inhibition of proteins by oxidation of their drial ROS production either as a superoxide or as a hydrogen thiol group, some of the signaling pathways become unblocked peroxide, most of them non-mammalian: external NADH dehy- and therefore active. This is specifically true for the inactivation drogenase (Fang and Beattie, 2003b) or alternative oxidase (Fang of protein phosphatases by H2O2, thereby increasing the level of and Beattie, 2003a), proline dehydrogenase (White et al., 2007), protein phosphorylation (Meng et al., 2002). In contrast, redox cytochrome b5 reductase (Whatley et al., 1998), monoamine oxi- dependent inactivation of protein tyrosine phosphatases may be dase (Koopman et al., 2010) and dihydroorotate dehydrogenase specific and reversible (Denu and Tanner, 1998). ROS can also (Forman and Kennedy, 1976) as well as potentially complex IV directly affect kinase signaling, for example a receptor tyrosine (Koopman